Twin wire former

ABSTRACT

A twin wire former in a machine for the production of a fibrous material web, specifically a paper or cardboard web, includes two rotating continuous wires which meet in the area of a simultaneously rotating dewatering element, thereby forming a stock inlet gap; and a headbox over which the fiber stock suspension is fed into the stock inlet gap. The stock consistency C of the fiber stock suspension in the headbox, as well as the basis weight F of the fiber stock suspension supplied to the stock inlet gap is selected according to the calculation: 
       F/ ( C *1000)&gt;0.025 
     whereby the basis weight F is stated in g/m 2  and the stock consistency C is stated in g/l.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention.

[0002] The present invention relates to a twin wire former for theproduction of a fibrous material web, specifically a paper or cardboardweb. 2. Description of the Related Art

[0003] A twin wire former having two rotating continuous wires whichmeet in the area of a simultaneously rotating dewatering element whichthereby form a stock inlet gap; and a headbox from which the fiber stocksuspension is fed into the stock inlet gap is described in PCTpublication WO 97/47803.

SUMMARY OF THE INVENTION

[0004] The present invention provides a twin wire former which providesthe highest possible web strength, soft-flake web structure, and anoptimal streak-free formation.

[0005] The consistency of packaging paper and cardboard is dependentupon the volume of water which is removed on the forming roll of aformer. Contrary to theoretical assumptions it has been demonstratedthat a larger forming roll diameter with the same angle of wrap resultsin a higher dewatering capacity than a smaller diameter forming roll.

[0006] As the forming roll diameter is increased, the maximum thicknessof the suspension jet exiting the headbox may also increase, withoutconcern for backflows at the stock inlet gap.

[0007] Additionally, the achievable dimensional ratio (L/h) between jetlength L and stream height h decreases with increasing suspension jetthickness which manifests itself in a streak-free formation, withoutsignificant influence of the boundary layer turbulence of the headboxwalls.

[0008] The current invention creates a twin wire former of the typedescribed at the beginning of this document with which, underconsideration of the aforementioned factors the highest possible webstrength, an optimum streak-free formation and an optimum soft-flake webstructure is achieved.

[0009] According to the invention, the stock consistency C of thefibrous stock suspension in the headbox, as well as the basis weight Fof the fibrous stock suspension supplied into the stock inlet gap isselected according to the calculation:

F/(C*1000)>0.025

[0010] whereby the basis weight F is stated in g/m² and the stockconsistency C is stated in g/l.

[0011] The ratio between the maximum length of the suspension jetflowing between the discharge slice of the headbox and the stock inletgap and the thickness of the free suspension jet is preferably smallerthan 10.

[0012] If the discharge slice of the headbox is located, for examplebetween two nozzle walls and/or one or more separating elements whichare positioned transversely to the direction of the stream and one isoffset from the other, then the maximum length of the free suspensionjet may be determined by the distance between the set back nozzle walland the point at which the suspension jet segment, on the side of theoffset nozzle wall, impacts the relevant wire.

[0013] The dewatering element may for example be formed by a roll or bya rotating belt or fabric which would preferably be guided over curvedelements. The respective roll may have an open or closed surface. It maybe operated with or without vacuum. The respective belt or fabric may bespecifically an open belt or fabric.

[0014] One or more formation elements and/or one or more dewateringelements may be provided following the dewatering or forming element.

[0015] In a suitable, practical embodiment of the twin wire formeraccording to the invention, the dewatering element around which bothwires wrap, has a curvature radius in the wrap-around area which isequal to or larger than approximately 900 mm and preferably larger thanapproximately 1000 mm. If the dewatering element is a roll, then theroll diameter is preferably greater than or equal to approximately 1800mm, and more preferably greater than approximately 2000 mm.

[0016] It is also an advantage if the convergence angle which is formedbetween one of the two transverse nozzle walls and the direction of jetflow is greater than or equal to approximately 1°.

[0017] On one of the curved dewatering elements wrapped by the twowires, the ratio between the radius of curvature and the thickness ofthe free suspension jet is preferably less than approximately 45, andmore preferably less than approximately 35.

[0018] It is also an advantage if the length of wrap X over which thetwo wires wrap around the dewatering element, and the thickness h of thefree suspension jet are selected so that the value resulting from theequation {square root}{square root over ((X*h))} is in the range ofapproximately 140 mm to approximately 300 mm and preferably in a rangeof between approximately 160 mm and approximately 300 mm.

[0019] The wire speed v, the wire tension T and the density ρ of thefiber stock suspension are appropriately designed to result in thefollowing: $\frac{T}{h*\rho*v^{2}} > 1$

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above-mentioned and other features and advantages of thisinvention, and the manner of attaining them, will become more apparentand the invention will be better understood by reference to thefollowing description of an embodiment of the invention taken inconjunction with the accompanying drawing, wherein:

[0021]FIG. 1 is a schematic illustration of an embodiment of a twin wireformer of the present invention.

[0022] Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplification set out hereinillustrates one preferred embodiment of the invention, in one form, andsuch exemplification is not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

[0023] Referring now to the drawing, and more particularly to FIG. 1,there is shown a schematic drawing of an embodiment of a twin wireformer 10 for the production of a fibrous material web 12, which mayspecifically be paper or cardboard.

[0024] Twin wire former 10 includes two rotating continuous wires 14 and16 which meet in the area of a simultaneously rotating dewatering orforming element 20, the meeting of which form a stock inlet gap.Dewatering element 20 may for example be a forming roll or a formingbelt or fabric which is supported by blades.

[0025] Outer wire 14 which does not come into direct contact withdewatering element 20 is led over a breast roll 22 in the area of stockinlet gap 18.

[0026] Fiber stock suspension 12′ is supplied to stock inlet gap 18 fromheadbox 24.

[0027] Along the twin wire travel path 26, following dewatering element20, which in this example is a forming roll, additional elements 28 and30 assist in the forming and/or the dewatering process.

[0028] Following element 30, outer wire 14 travels over turning roll 32,which effectively separates it from inside wire 16 which continues tosupport fiber stock web 12. In order to ensure that fiber stock web 12remains adhered to inside wire 16 and is appropriately transported, anadditional separating element may be provided.

[0029] Outer wire 14 is returned to breast roll 22 by running over anadditional turning roll 36.

[0030] The stock consistency C of fiber suspension 12′ in headbox 24, aswell as the basis weight F of fiber suspension 12′ supplied to stockinlet gap 18 is selected according to the following formula:

F/(C*1000)>0.025

[0031] whereby the basis weight F is expressed in g/m² and the stockconsistency C in g/l.

[0032] The ratio between the maximum length L of the suspension jetflowing freely between discharge slice 38 of headbox 24 and stock inletgap 18, and the thickness h of free suspension jet 40 is preferably lessthan 20 and more preferably less than 10.

[0033] Discharge slice 38 of headbox 24 is located for example betweennozzle wall 42 and nozzle wall 44 which are positioned transversely tothe direction of travel of stream S whereby the top nozzle wall 44 isoffset from the bottom nozzle wall 42. The maximum length L of freesuspension jet 40 may be determined by the distance between nozzle wall44 and the point of impact A at which the suspension jet segment on theside of top nozzle wall 44 impacts outer wire 14.

[0034] Dewatering element 20 around which both wires 14 and 16 wrap, hasa curvature radius R in the wrap around area X, which is effectivelygreater than or equal to approximately 900 mm and preferably greaterthan approximately 1000 mm. In the present example the dewateringelement 20 is a roll. The roll diameter D is therefore effectivelygreater than or equal to approximately 1800 mm, and preferably greaterthan approximately 2000 mm.

[0035] The convergence angle α formed between top nozzle wall 44 and thedirection of the jet flow S is preferably larger than or equal toapproximately 1°.

[0036] The ratio between the curvature radius R of the dewateringelement 20 in the wrap-around area X and the thickness h of the freesuspension jet 40 is preferably less than approximately 45 and morepreferably less than approximately 35. Since dewatering element 20 inthe present example features a roll, the curvature radius R is equal tothe roll radius.

[0037] The length of the wrap X over which the two wires 14, 16 wraparound dewatering element 20, and the thickness h of free suspension jet40 are selected preferably so that the value resulting from the equation{square root}{square root over ((X*h))} is in the range of approximately140 mm to approximately 300 mm and preferably in a range of betweenapproximately 160 mm and approximately 300 mm.

[0038] The wire speed v, the wire tension T and the density ρ of fiberstock suspension 12′ are effectively designed to result in the followingrelationship: $\frac{T}{h*\rho*v^{2}} > 1$

[0039] While this invention has been described as having a preferreddesign, the present invention can be further modified within the spiritand scope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

Component Identification Index

[0040]10 Twin wire former

[0041]12 Fiber stock web

[0042]12′ Fiber stock suspension

[0043]14 Outer wire

[0044]16 Innerwire

[0045]18 Stock inlet gap

[0046]20 Simultaneously rotating dewatering element

[0047]22 Breast roll

[0048]24 Twinwire

[0049]26 Twin wire travel path

[0050]28 Forming or dewatering element

[0051]30 Forming or dewatering element

[0052]32 Turning roller

[0053]34 Separating element

[0054]36 Turning roller

[0055]38 Discharge slice

[0056]40 Free suspension jet

[0057]2 Nozzle wall

[0058]44 Nozzle wall

[0059] h Thickness of the free suspension jet

[0060] A Point of impact

[0061] D Roll diameter

[0062] L Maximum length of the free suspension jet

[0063] R Curvature radius, roll radius

[0064] S Direction of jet flow

[0065] x Wrap-around area, wrap-around length.

What is claimed is:
 1. A twin wire former for the production of afibrous material web, comprising: a rotating dewatering element; a setof two counter rotating continuous wires meeting in an area of saidrotating dewatering element and forming a stock inlet gap therebetween;a headbox including a discharge slice, said headbox configured to feed afiber stock suspension through said discharge slice into said stockinlet gap; a source of fiber stock suspension to be supplied to saidheadbox selected according to the calculation: F/(C*1000)>0.025where Fis the basis weight in g/m² of the fiber stock suspension supplied tosaid stock inlet gap and C is the stock consistency in g/l of the fiberstock suspension in said headbox.
 2. The twin wire former of claim 1,wherein said headbox is adjustable such that the ratio of the maximumlength L of a suspension jet flowing from said discharge slice and athickness h of the suspension jet is preferably less than
 20. 3. Thetwin wire former of claim 2, wherein said ratio of L to h is less than10.
 4. The twin wire former of claim 2, wherein said headbox includes anupper nozzle wall and a lower nozzle wall positioned transversely to thedirection of flow of the suspension jet and defining said dischargeslice therebetween, said lower nozzle wall being offset relative to saidupper nozzle wall whereby L is determined by the distance from saidupper nozzle wall and the point of impact of the top of the suspensionjet with one of said continuous wires.
 5. The twin wire former of claim2, wherein said headbox includes an upper nozzle wall and a lower nozzlewall positioned transversely to the direction of flow of the suspensionjet and defining said discharge slice therebetween, said upper nozzlewall being configured such that the angle α formed between thesuspension jet and said upper nozzle wall is greater than or equal toapproximately 1°.
 6. The twin wire former of claim 2, wherein saidrotating dewatering element is curved, having a curvature radius R, adistance said continuous wires have contact with said rotatingdewatering element being X and the ratio of R to h being less thanapproximately
 45. 7. The twin wire former of claim 6, wherein said ratioof R to h is less than approximately
 35. 8. The twin wire former ofclaim 2, wherein a length over which said continuous wires have contactwith said rotating dewatering element X and h are selected so that thevalue resulting from the equation {square root}{square root over((X*h))} is in the range of between approximately 140 mm andapproximately 300 mm.
 9. The twin wire former of claim 8, wherein thevalue resulting from said equation {square root}{square root over((X*h))} is in the range of between approximately 160 mm andapproximately 300 mm.
 10. The twin wire former of claim 2, wherein atraveling velocity v of said continuous wires, a tension T of saidcontinuous wires, a density ρ of the fiber stock suspension and hselected such that the following equation is true:$\frac{T}{h*\rho*v^{2}} > 1.$


11. The twin wire former of claim 1, wherein said rotating dewateringelement is a roll.
 12. The twin wire former of claim 1, wherein saidrotating dewatering element includes at least one curved element and arevolving belt running over said at least one curved element.
 13. Thetwin wire former of claim 1, wherein said rotating dewatering element iscurved having a curvature radius R effectively greater than or equal toapproximately 900 mm.
 14. The twin wire former of claim 13, wherein saidcurvature radius R is greater than approximately 1,000 mm.
 15. The twinwire former of claim 1, wherein said rotating dewatering element is aroll of diameter D greater than or equal to approximately 1800 mm. 16.The twin wire former of claim 15, wherein said diameter D is greaterthan approximately 2,000 mm.
 17. A method of producing a fibrousmaterial web using a twin wire former, said method comprising the stepsof: providing a twin wire former having a rotating dewatering element, aset of two counter rotating continuous wires defining a stock inlet gaptherebetween, and a headbox including a discharge slice; selecting asource of fiber stock suspension to be supplied to said headboxaccording to the calculation: F/(C*1000)>0.025where F is the basisweight in g/m² of the fiber stock suspension supplied to said stockinlet gap and C is the stock consistency in g/l of the fiber stocksuspension in said headbox; feeding the fiber stock suspension throughsaid discharge slice into said stock inlet gap; and operating said twinwire former using said fiber stock suspension to produce the fibrousmaterial web.
 18. The method of producing a fibrous material web using atwin wire former of claim 17, including the steps of: selecting athickness h of the suspension jet, a velocity v of said continuouswires, a tension T of said continuous wires, and a density p of thefiber stock suspension such that the following equation is true:$\frac{T}{h*\rho*v^{2}} > 1$